ceph/src/boost/libs/fiber/doc/html/fiber/rationale.html

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  25 <div class="section">
  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="fiber.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
  28 </h2></div></div></div>
  29 <h4>
  30 <a name="fiber.rationale.h0"></a>
  31       <span><a name="fiber.rationale.distinction_between_coroutines_and_fibers"></a></span><a class="link" href="rationale.html#fiber.rationale.distinction_between_coroutines_and_fibers">distinction
  32       between coroutines and fibers</a>
  33     </h4>
  34 <p>
  35       The fiber library extends the coroutine library by adding a scheduler and synchronization
  36       mechanisms.
  37     </p>
  38 <div class="itemizedlist"><ul class="itemizedlist" type="disc">
  39 <li class="listitem">
  40           a coroutine yields
  41         </li>
  42 <li class="listitem">
  43           a fiber blocks
  44         </li>
  45 </ul></div>
  46 <p>
  47       When a coroutine yields, it passes control directly to its caller (or, in the
  48       case of symmetric coroutines, a designated other coroutine). When a fiber blocks,
  49       it implicitly passes control to the fiber scheduler. Coroutines have no scheduler
  50       because they need no scheduler.<sup>[<a name="fiber.rationale.f0" href="#ftn.fiber.rationale.f0" class="footnote">10</a>]</sup>.
  51     </p>
  52 <h4>
  53 <a name="fiber.rationale.h1"></a>
  54       <span><a name="fiber.rationale.what_about_transactional_memory"></a></span><a class="link" href="rationale.html#fiber.rationale.what_about_transactional_memory">what
  55       about transactional memory</a>
  56     </h4>
  57 <p>
  58       GCC supports transactional memory since version 4.7. Unfortunately tests show
  59       that transactional memory is slower (ca. 4x) than spinlocks using atomics.
  60       Once transactional memory is improved (supporting hybrid tm), spinlocks will
  61       be replaced by __transaction_atomic{} statements surrounding the critical sections.
  62     </p>
  63 <h4>
  64 <a name="fiber.rationale.h2"></a>
  65       <span><a name="fiber.rationale.synchronization_between_fibers_running_in_different_threads"></a></span><a class="link" href="rationale.html#fiber.rationale.synchronization_between_fibers_running_in_different_threads">synchronization
  66       between fibers running in different threads</a>
  67     </h4>
  68 <p>
  69       Synchronization classes from <a href="http://www.boost.org/doc/libs/release/libs/thread/index.html" target="_top">Boost.Thread</a>
  70       block the entire thread. In contrast, the synchronization classes from <span class="bold"><strong>Boost.Fiber</strong></span> block only specific fibers, so that the
  71       scheduler can still keep the thread busy running other fibers in the meantime.
  72       The synchronization classes from <span class="bold"><strong>Boost.Fiber</strong></span>
  73       are designed to be thread-safe, i.e. it is possible to synchronize fibers running
  74       in different threads as well as fibers running in the same thread. (However,
  75       there is a build option to disable cross-thread fiber synchronization support;
  76       see <a class="link" href="overview.html#cross_thread_sync">this description</a>.)
  77     </p>
  78 <a name="spurious_wakeup"></a><h4>
  79 <a name="fiber.rationale.h3"></a>
  80       <span><a name="fiber.rationale.spurious_wakeup"></a></span><a class="link" href="rationale.html#fiber.rationale.spurious_wakeup">spurious
  81       wakeup</a>
  82     </h4>
  83 <p>
  84       Spurious wakeup can happen when using <a href="http://en.cppreference.com/w/cpp/thread/condition_variable" target="_top"><code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">condition_variable</span></code></a>:
  85       the condition variable appears to be have been signaled while the awaited condition
  86       may still be false. Spurious wakeup can happen repeatedly and is caused on
  87       some multiprocessor systems where making <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">condition_variable</span></code>
  88       wakeup completely predictable would slow down all <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">condition_variable</span></code>
  89       operations.<sup>[<a name="fiber.rationale.f1" href="#ftn.fiber.rationale.f1" class="footnote">11</a>]</sup>
  90     </p>
  91 <p>
  92       <a class="link" href="synchronization/conditions.html#class_condition_variable"><code class="computeroutput">condition_variable</code></a> is not subject to spurious wakeup.
  93       Nonetheless it is prudent to test the business-logic condition in a <code class="computeroutput"><span class="identifier">wait</span><span class="special">()</span></code> loop
  94       &#8212; or, equivalently, use one of the <code class="computeroutput"><span class="identifier">wait</span><span class="special">(</span> <span class="identifier">lock</span><span class="special">,</span>
  95       <span class="identifier">predicate</span> <span class="special">)</span></code>
  96       overloads.
  97     </p>
  98 <p>
  99       See also <a class="link" href="synchronization/conditions.html#condition_variable_spurious_wakeups">No Spurious Wakeups</a>.
 100     </p>
 101 <h4>
 102 <a name="fiber.rationale.h4"></a>
 103       <span><a name="fiber.rationale.migrating_fibers_between_threads"></a></span><a class="link" href="rationale.html#fiber.rationale.migrating_fibers_between_threads">migrating
 104       fibers between threads</a>
 105     </h4>
 106 <p>
 107       Support for migrating fibers between threads has been integrated. The user-defined
 108       scheduler must call <a class="link" href="scheduling.html#context_detach"><code class="computeroutput">context::detach()</code></a> on a fiber-context on the
 109       source thread and <a class="link" href="scheduling.html#context_attach"><code class="computeroutput">context::attach()</code></a> on the destination thread,
 110       passing the fiber-context to migrate. (For more information about custom schedulers,
 111       see <a class="link" href="custom.html#custom">Customization</a>.) Examples <code class="computeroutput"><span class="identifier">work_sharing</span></code>
 112       and <code class="computeroutput"><span class="identifier">work_stealing</span></code> in directory
 113       <code class="computeroutput"><span class="identifier">examples</span></code> might be used as a
 114       blueprint.
 115     </p>
 116 <p>
 117       See also <a class="link" href="migration.html#migration">Migrating fibers between threads</a>.
 118     </p>
 119 <h4>
 120 <a name="fiber.rationale.h5"></a>
 121       <span><a name="fiber.rationale.support_for_boost_asio"></a></span><a class="link" href="rationale.html#fiber.rationale.support_for_boost_asio">support
 122       for Boost.Asio</a>
 123     </h4>
 124 <p>
 125       Support for <a href="http://www.boost.org/doc/libs/release/libs/asio/index.html" target="_top">Boost.Asio</a>&#8217;s
 126       <span class="emphasis"><em>async-result</em></span> is not part of the official API. However,
 127       to integrate with a <a href="http://www.boost.org/doc/libs/release/doc/html/boost_asio/reference/io_service.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_service</span></code></a>,
 128       see <a class="link" href="integration.html#integration">Sharing a Thread with Another Main Loop</a>.
 129       To interface smoothly with an arbitrary Asio async I/O operation, see <a class="link" href="callbacks/then_there_s____boost_asio__.html#callbacks_asio">Then There&#8217;s <a href="http://www.boost.org/doc/libs/release/libs/asio/index.html" target="_top">Boost.Asio</a></a>.
 130     </p>
 131 <h4>
 132 <a name="fiber.rationale.h6"></a>
 133       <span><a name="fiber.rationale.tested_compilers"></a></span><a class="link" href="rationale.html#fiber.rationale.tested_compilers">tested
 134       compilers</a>
 135     </h4>
 136 <p>
 137       The library was tested with GCC-5.1.1, Clang-3.6.0 and MSVC-14.0 in c++11-mode.
 138     </p>
 139 <h4>
 140 <a name="fiber.rationale.h7"></a>
 141       <span><a name="fiber.rationale.supported_architectures"></a></span><a class="link" href="rationale.html#fiber.rationale.supported_architectures">supported
 142       architectures</a>
 143     </h4>
 144 <p>
 145       <span class="bold"><strong>Boost.Fiber</strong></span> depends on <a href="http://www.boost.org/doc/libs/release/libs/context/index.html" target="_top">Boost.Context</a>
 146       - the list of supported architectures can be found <a href="http://www.boost.org/doc/libs/release/libs/context/doc/html/context/architectures.html" target="_top">here</a>.
 147     </p>
 148 <div class="footnotes">
 149 <br><hr width="100" align="left">
 150 <div class="footnote"><p><sup>[<a name="ftn.fiber.rationale.f0" href="#fiber.rationale.f0" class="para">10</a>] </sup>
 151         <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4024.pdf" target="_top">'N4024:
 152         Distinguishing coroutines and fibers'</a>
 153       </p></div>
 154 <div class="footnote"><p><sup>[<a name="ftn.fiber.rationale.f1" href="#fiber.rationale.f1" class="para">11</a>] </sup>
 155         David R. Butenhof <span class="quote">&#8220;<span class="quote">Programming with POSIX Threads</span>&#8221;</span>
 156       </p></div>
 157 </div>
 158 </div>
 159 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 160 <td align="left"></td>
 161 <td align="right"><div class="copyright-footer">Copyright &#169; 2013 Oliver Kowalke<p>
 162         Distributed under the Boost Software License, Version 1.0. (See accompanying
 163         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
 164       </p>
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